Conventionally, as a rectilinear guide unit of the above type, there is known the one that has a structure comprising a track rail arranged on a stationary portion such as a bed and having ball rolling surface and a slider fastened to the track rail through a number of balls and movable along the rail while supporting a movable body such as a table.
Further, the slider comprises a movable block having ball load rolling surfaces opposing to the ball rolling surfaces of the track rail through the balls and ball return holes parallel to the ball load rolling surfaces and capable of moving along the track rail following the rolling of the balls and a pair of covers having change-direction paths for establishing communication between the ball load rolling surfaces and the ball return holes, and by fixing the covers to both front and rear end surfaces of the movable block, respectively, the load rolling surfaces and the ends of the ball return holes are respectively connected by the change direction paths thereby completing in the slider an endless circulation path for the balls.
In the case of the slider of the conventional rectilinear guide unit having the above-described structure, the process of drilling the ball return holes and the fixing of the ball retainers to the movable block becomes necessary resulting in taking much time and labor for its manufacture so that the present applicant provided such slider by making use of extrusion molding of a synthetic resin (refer to Unexamined Published Japanese Patent Application No. 7-317762).
FIG. 12 shows one example of a slider of a rectilinear guide unit manufactured according to the above-described method wherein the slider 101 fastened to a track rail 100 also comprises a movable block 102 and a pair of covers 103, 103 such that the movable block 102 comprises a metallic block main body 105 having load rolling surfaces 104 and padded with a synthetic resin by an injection-molding method and as shown in FIG. 13, ball return holes 107 corresponding to the load rolling surfaces 104 are formed in a pair of side surface molded portions 106 padded to the outer lower side surfaces of the block main body 105, respectively. Further, to simplify the shape of each of the covers 13, there are padded to the front and rear end surfaces of the block main body 105 a pair of end surface molded portions 108 which are continuous with the side surface molded portions 106, respectively, as shown in FIG. 14 and these molded portions 108 are provided with semicircular ball guide sections (not shown) for guiding the balls which have rolled the load rolling surfaces 104 to the ball return holes 107.
According to such slider manufacturing method making use of the injection molding of synthetic resin material, it is possible to manufacture in a simple manner the movable block 102 of a complicated shape by merely padding a synthetic resin over the block main body 105 and to continuously form the ball return holes 107 and the change direction paths through which unloaded balls roll with the advantage that the circulation of the balls is smoothed and the noise level at the time of rolling of the balls is reduced.
Now, due to the fact that the synthetic resin mold produced by injection-molding contracts in the direction of the large thickness portion thereof, when the side surface molded portions 106 are padded to the outer side surfaces of the block main body 105 so as to form the above-mentioned ball return holes 107 in the side surface molded portions 106, the portions 106 contract in the longitudinal direction thereof, that is, in a direction parallel to the ball return holes 107.
However, since both end surfaces of the block main body 105 are also provided with the synthetic resin end surface molded portions 108 padded by injection-molding and formed integral with the side surface molded portions 106 respectively, when the molded portions 106 contract for the above-described reason, the forces of such contraction of the molded portions 106 act unevenly upon the end surface molded portions 108.
Consequently, there has arisen a problem that as shown in FIG. 14, after a little while from the completion of injection-molding, the end surface molded portions 108 are pulled by the side surface molded portions 106 to become deformed and float up from the end surfaces of the block main body 105 so that gaps are formed between the block main body 105 and the end surface molded portions 108, respectively.
Further, there has also arisen a problem that although the covers 103 are respectively fixed to the end surface molded portions 108 to allow the slider 101 to have the change direction paths for the balls, when each of the end surface molded portions 18 deforms, the covers 103 can no more be fixed tightly to the end surface molded portions 108 failing to accurately form the change direction paths so that the balls rolled on the load rolling surface 104 can not be smoothly guided with respect to the ball return holes 107.